SMT007 Magazine

SMT-Oct2017

Contents of this Issue

Navigation

Page 67 of 97

68 SMT Magazine • October 2017
lead-free solders and testing of individual solder
ball attachments rather than the advanced ball
grid arrays (BGAs) electronic packaging assem-
blies and for tin-lead solder. It is extremely dif-
ficult to correlate test results from the shear of
individual balls to the assemblies of BGAs. Re-
liability testing using BGA packages, especially
land grid arrays, with tin-lead solder is lacking.
In addition, controversy exists regarding the re-
liability/compatibility of ENEPIG with tin-lead
solder
4
, as there are more consistent positive test
results for the lead-free solder joints
5
. A hot air
solder level (HASL) finish, which is commonly
used for the tin-lead solder, lacks the flatness
requirement for the fine pitch ball grid arrays
(FBGAs).
The significance of the PCB surface finish on
assembly reliability was recognized by the IPC
9701 standard team in its early development,
when the team was narrowing the requirements
for the tin-lead solder joint testing for the BGA
packaging technologies. The team limited the
use of the PCB finishes to solder preservative
(OSP) and HASL to minimize the potential for
premature failures by using other finishes. Such
restriction was implemented to avoid significant
cost and schedule burden on smaller facilities,
which generally lack knowledge and experience
on the nuances of unique surface finishes such
as electroless nickel/immersion gold (ENIG).
This specification allowed unique surface fin-
ishes only for comparison to the baseline finish.
The A revision, which also includes recommen-
dations for Pb-free solders, allowed only the use
of an OSP finish since the tin-lead HASL was not
compatible to Pb-free solders. Other surface fin-
ishes including silver (Ag) and tin (Sn) were ac-
ceptable only for a manufacturer's internal data
comparison. Also, an ENIG finish could be used
for an internal data comparison; however, it was
warned that the risk of introducing unintended
brittle failure (black pad) could occur. The EN-
EPIG was not introduced at this time; therefore,
this specification does not discuss this specific
surface finish.
With the industry implementation of Re-
striction of Hazardous Substances (RoHS), the
use of a tin-lead HASL finish for PCB with excel-
lent solderability and solder joint reliability has
diminished even though this still is the domi-
nant finish for high-reliability applications with
tin-lead solder. The process consists of immers-
ing PCBs in a tin-lead alloy followed by solder
removal using air knives, which blow hot air
across the surface of the PCB [to remove excess
solder]. The lead-free HASL finish has gained
some interest for RoHS use, but it suffers from
increased copper dissolution and lacks the flat-
ness requirement needed for finer pitch array
packages. An ENIG finish provides the flatness
requirements with excellent solderability, but
it suffers from the "black pad" potential failure
and lacks gold wire bondability that is required
for hybrid (wire and solder) technologies
6
.
ENEPIG has provided the best solution for
the black pad defect by depositing an additional
layer of electroless palladium over nickel. The
palladium is not etched away during the gold
plating process so the potential for the oxida
-
tion of nickel (black pad) is eliminated. However,
ENEPIG is costlier than ENIG. ENEPIG also pro-
vides excellent solder joint reliability for lead-
free solder joints, but industry debates continue
on its reliability with tin-lead solder assembly.
The IPC-4556 specification for ENEPIG is
very comprehensive and includes a wealth of
information. The thickness specification for
ENEPIG specified as: 1) Nickel: 3 to 6 μm (118.1
to 236.2 μin), 2) Palladium: 0.05 to 0.15 μm (2
to 12 μin), and 3) Gold: a minimum thickness
of 0.030 μm (1.2 μin). All measurements are to
be taken on a nominal pad size of 1.5 mm x
1.5 mm (0.060 in x 0.060 in) or equivalent area.
The new amendment sets an upper limit for the
gold thickness at 2.8 μin (0.7 μm) to discour-
age requests for a much higher immersion gold
RELIABILITY OF ENEPIG BY SEQUENTIAL THERMAL CYCLING AND AGING
"
A hot air solder level (HASL)
finish, which is commonly used
for the tin-lead solder, lacks the
flatness requirement for the fine
pitch ball grid arrays (FBGAs).
"